Continuous low dose rate irradiation has been used successfully in intracavitary and interstitial implants. Its superiority over conventional fractionated beam therapy has been suggested but not demonstrated. The objective of this project is to gain a better insght into cellular kinetics under continuous irradiation and to compare it with fractionated regimens in a simple in vitro system. In the first stage of the study, the dose rate effect due primarily to accumulation and repair of radiation damage was investigated by continuous irradiation of cells through mitotic cycle. The main emphasis during the second year will be on dose rate effects in cells with different mitotic cycle times, and comparison of continuous and fractionated irradiation at temperatures allowing cell division.